Plants specifically improved for agriculture, horticulture, biomass conversion, and other industries (e.g. paper industry, plants as production factories for proteins or other compounds) can be obtained using molecular technologies. As an example, great agronomic value can result from enhancing plant growth in saline and/or oxidative stress conditions.
Salinity
A wide variety agriculturally important plant species demonstrate significant sensitivity to saline and/or oxidative stress conditions. Upon salt concentration exceeding a relatively low threshold, many plants suffer from stunted growth, necrosis, and death that results in an overall stunted appearance and reduced yields of plant material, seeds, fruit and other valuable products. Physiologically, plants challenged with salinity experience disruption in ion and water homeostasis, inhibition of metabolism, and damage to cellular membranes that result in developmental arrest and cell death (Huh et al. (2002) Plant J, 29(5):649-59).
In many of the world's most productive agricultural regions, agricultural activities themselves lead to increased water and soil salinity, which threatens their sustained productivity. One example is crop irrigation in arid regions that have abundant sunlight. After irrigation water is applied to cropland, it is removed by the processes of evaporation and transpiration. While these processes remove water from the soil, they leave behind dissolved salts carried in irrigation water. Consequently, soil and groundwater salt concentrations build over time, rendering the land and shallow groundwater saline and thus damaging to crops.
In addition to human activities, natural geological processes have created vast tracts of saline land that would be highly productive if not saline. In total, approximately 20% of the irrigated lands are negatively affected by salinity. (Yamaguchi and Blumwald, 2005, Trends in Plant Science, 10: 615-620). For these and other reasons, it is of great interest and importance to identify genes that confer improved salt tolerance characteristics to thereby enable one to create transgenic plants (such as crop plants) with enhanced growth and/or productivity characteristics in saline conditions.
Despite this progress, today there continues to be a great need for generally applicable processes that improve forest or agricultural plant growth to suit particular needs depending on specific environmental conditions. To this end, the present invention is directed to advantageously manipulating plant tolerance to salinity in order to maximize the benefits of various crops depending on the benefit sought, and is characterized by expression of recombinant DNA molecules in plants. These molecules may be from the plant itself, and simply expressed at a higher or lower level, or the molecules may be from different plant species.
Oxidative Stress
Plants lead a sessile lifestyle and so are generally destined to reside where their seed germinates. Consequently, they can be exposed to unfavorable environmental conditions arising from weather, pollution and location. Stress conditions, such as extremes in temperature, drought and desiccation, salinity, soil nutrient content, heavy metals, UV radiation, pollutants such as ozone and SO2, mechanical stress, high light and pathogen attack, have a large impact on plant growth and development. These types of stress exposure induce formation of toxic oxygen species, which are generated in all aerobic cells and are associated with oxidative damage at the cellular level. Several recently published reports have characterized toxic oxygen species generation and the subsequent oxidative damage caused by abiotic stresses (see Larkindale and Knight (2002); Borsani et al. (2001); Lee et al (2004); Aroca et al (2005); Luna et al (2005); and Noctor et al (2002)).
The toxic oxygen species are referred to as reactive oxygen species (ROS), reactive oxygen intermediates (ROI) or activated oxygen species (AOS) and are partially reduced or activated derivatives of oxygen. ROS/ROI/AOS include the oxygen-centered superoxide (O2) and hydroxyl (.OH) free radicals as well as hydrogen peroxide (H2O2), nitric oxide (NO) and O21. These oxygen species are generated as byproducts from reactions that occur during photosynthesis, respiration and photorespiration, and are predominantly formed in the chloroplasts, mitochondria, endoplasmic reticulum, microbodies (e.g. peroxisomes and glyoxysomes), plasma membranes and cell walls. While the toxicity of O2− and H2O2 themselves is relatively low, their metal-dependent conversion to highly toxic .OH is thought to be responsible for the majority of the biological damage associated with these molecules.
Oxidative stress damages cell structure and affects cell metabolism and catabolism. Membrane lipids are subject to oxidation by ROS/ROI/AOS, resulting in accumulation of high molecular weight, cross-linked fatty acids and phospholipids. Oxidative attack on proteins results in site-specific amino acid modifications, fragmentation of the peptide chain, aggregation of cross-linked reaction products, altered electrical charge and increased susceptibility to proteolysis, all of which frequently leads to elimination of enzyme activity. ROS/ROI/AOS that generate oxygen free radicals, such as ionizing radiation, also induce numerous lesions in DNA at both the sugar and base moieties which cause deletions, mutation and other lethal genetic effects such as base degradation, single strand breakage and cross-linking to proteins. Morphologically, the adverse effects of high levels of ROS accumulation are manifested as stunted growth and necrotic lesions.
Although capable of producing damage, ROS/ROI/AOS are also key regulators of metabolic and defense pathways, playing roles as signaling or secondary messenger molecules. For example, pathogen-induced ROS/ROI/AOS production is critical in disease resistance where these molecules are involved at three different levels: penetration resistance, hypersensitive response (HR) and systemic acquired resistance (Levine et al. (1994); Lamb and Dixon (1997); Zhou et al. (2000); Aviv et al. (2002)). In penetration resistance, ROS/ROI/AOS function by reinforcing cell walls through polyphenolic cross-linking. With respect to hypersensitive response, H2O2 is an active signaling molecule whose effect is dose dependent. At high dosages, H2O2 triggers hypersensitive cell death and thus restricts the pathogen to local infection sites (Lamb and Dixon (1997)) while low dosages block cell cycle progression (Reichheld et al. (1999)) and signal secondary wall differentiation (Potikha et al. (1999)). Lastly, ROS/ROI/AOS molecules play a role in broad-spectrum systemic acquired disease resistance by triggering micro-HR systematically after the first pathogen inoculation.
In the signal cascades leading to oxidative stress, salicylic acid (SA) has been identified as an important signaling molecule to mediate ROS/ROI/AOS accumulation in various stress conditions, such as salt and osmotic stress (Borsani et al. (2001)), drought (Senaratna et al. (2000)), heat (Dat et al. (1998)), cold (Scott et al. (2004)), UV-light (Surplus et al. (1998)), paraquat (Kim et al. (2003)) and disease resistance against different pathogens (Zhou et al. (2004)). High levels of SA induce H2O2 production as well as cell death.
Several signaling components required for SA-mediated ROS/ROI/AOS accumulation and gene expression have been characterized. For example, NPR1 is required for SA-induced PR gene expression and disease resistance (Cao et al. (1994)). The mutations in eds1 and eds5 block SA-mediated signaling and enhance disease susceptibility (Rusterucci et al. (2001)). Over-expression of NahG in various plant species also suppresses SA-induced responses to both abiotic and biotic stresses (Delaney et al. (1994)). Recently, Scott and colleagues (2004) reported that chilling treatment induced accumulation of SA in Arabidopsis and the degradation of SA by overexpression of NahG enhanced cold tolerance in a transgenic plant.
SA, as a phytohormone, also promotes early flowering (Martinez et al. (2004)). SA at various levels may play different roles in plant growth and stress responses. However, most of the time, the increased tolerance to high levels of SA appears to be beneficial, since it reduces the side effects of SA accumulation while stimulating SA-mediated stress responses.
Similarly, NO is capable of generating ROS/ROI/AOS and is a plant signaling molecule involved in the regulation of seed germination, stomatal closure (Mata and Lamattina (2001); Desikan et al (2002)), flowering time (He et al. (2004)), antioxidant reactions to suppress cell death (Beligni et al. (2002)) and tolerance to biotic and abiotic stress conditions (Mata and Lamattina (2001)). While the effects of NO can be mimicked through the application of sodium nitroprusside (SNP), endogenous NO production in plants results from the activity of a nitric oxide synthase that uses L-arginine (Guo et al. (2003)) as well as nitrate reductase-mediated reactions (Desikan et al (2002)). NO can react with redox centers in proteins and membranes, thereby causing cell damage and inducing cell death.
In order to control the two-fold nature of ROS/ROI/AOS molecules, plants have developed a sophisticated regulatory system which involves both production and scavenging of ROS/ROI/AOS in cells. During normal growth and development, this pathway monitors the level of ROS/ROI/AOS produced by metabolism and controls the expression and activity of ROS/ROI/AOS scavenging pathways. The major ROS/ROI/AOS scavenging mechanisms include the action of the superoxide dismutase (SOD), ascorbate perioxidase (APX) and catalase (CAT) enzymes as well as nonenzymatic components such as ascorbic acid, α-tocopherol and glutathione.
The antioxidant enzymes are believed to be critical components in preventing oxidative stress, in part because pretreatment of plants with one form of stress, and which induces expression of these enzymes, can increase tolerance for a different stress (cross-tolerance) Allen (1995)). In addition, plant lines selected for resistance to herbicides that function by inducing ROS/ROI/AOS generally have increased levels of one or more of these antioxidant enzymes and also exhibit cross-tolerance (Gressel and Galun (1994)).
Plant development and yield depend on the ability of the plant to manage oxidative stress, whether it is via the signaling or the scavenging pathways. Consequently, improvements in a plant's ability to withstand oxidative stress, or to obtain a higher degree of cross-tolerance once oxidative stress has been experienced, has significant value in agriculture. The sequences and methods of the invention provide the means by which tolerance to oxidative stress can be improved, either via the signaling or the scavenging pathways.
The availability and sustainability of a stream of food and feed for people and domesticated animals has been a high priority throughout the history of human civilization and lies at the origin of agriculture. Specialists and researchers in the fields of agronomy science, agriculture, crop science, horticulture, and forest science are even today constantly striving to find and produce plants with an increased growth potential to feed an increasing world population and to guarantee a supply of reproducible raw materials. The robust level of research in these fields of science indicates the level of importance leaders in every geographic environment and climate around the world place on providing sustainable sources of food, feed and energy.
Manipulation of crop performance has been accomplished conventionally for centuries through selection and plant breeding. The breeding process is, however, both time-consuming and labor-intensive. Furthermore, appropriate breeding programs must be specially designed for each relevant plant species.
On the other hand, great progress has been made in using molecular genetic approaches to manipulate plants to provide better crops. Through the introduction and expression of recombinant nucleic acid molecules in plants, researchers are now poised to provide the community with plant species tailored to grow more efficiently and yield more product despite suboptimal geographic and/or climatic environments. These new approaches have the additional advantage of not being limited to one plant species, but instead being applicable to multiple different plant species (Zhang et al. (2004) Plant Physiol. 135:615; Zhang et al. (2001) Proc. Natl. Acad. Sci. USA 98:12832).